Now, to the high-stakes george zimmerman murder trial. The prosecution's key witness back on the stand today. Trayvon martin's friend who was on the phone with him until seconds before he was shot. A chilling testimony about the 17-year-old's final moments. Abc's matt gutman is there in sanford, florida, with the latest. Good morning, matt. Reporter: Good morning, robin. Rachel jeantel sent 16 months as a mystery witness. Her identity kept hidden. This morning, she'll spend a few more hours on the stand. Yesterday, her testimony began with tears. And ended with her battling george zimmerman's attorneys in court. This morning, the prosecution's star witness, rachel jeantel, facing more crosation from george zimmerman's lawyer against his will. I'm leaving today. No. Are you refusing to come back tomorrow? To you? Reporter: Jeantel, once known as witness eight, was on the phone with her friend, 17-year-old trayvon martin, the night he was shot and killed by zimmerman. On wednesday, she told jurors martin thought he was being followed. He just told me the man, the man looked creepy. He said the man looked creepy? Creepy, white, execute my language, cracker. Reporter: She says she told martin to run away from zimmerman. And 18-year-old watchman -- you better keep running. Nah, he lost him. Reporter: But martin didn't lose him. Instead, jeantel heard the teen asking zimmerman -- why are you following me for? And I heard a hard breath man come say, what are you doing here. I started to hear a little bit of trayvon saying, get off. Get off. Reporter: Prosecutors believe jeantel's story proves that zimmerman followed and confronted martin and shot him. Zimmerman has pleaded not guilty, arguing martin assaulted him. And he only shot in self-defense. Jeantel's testimony, at times, hard to understand. But the emotional impact, undismakable. Defense attorneys challenging her on inconsistencies in her deposition. And why she lied about missing martin's wake. Under oath, was you created a lie and said you had gone to the hospital. Yes. Reporter: This morning, zimmerman's attorneys will suggest that jeantel has been lying to help martin's family. Was your purpose at that point to do what you could do, to assist to that george zimmerman got arrested? Yes. Reporter: And in less than two , she'll be back on the stand, facing those defense attorneys again. There may be a reason for them trying to rattle her. Remember, robin, the jury instructions are not only to listen to what the witness is saying but how they are saying it. Matt, thank you. We're going to bring in abc news chief legal affairs anchor, dan abrams. When you hear the testimony, i know you heard much of this key witness, what do you take away the most? She's a very important witness. But the most important thing that she said is that she says trayvon martin says get off, get off. Why is that so important? Because that would suggest that george zimmerman starts this incident. That's something george zimmerman says is not true. George zimmerman would say, trayvon martin effectively confronts him. So, if you believe her testimony, that becomes very important point, "a" in figuring out what happened, and, "b," in george zimmerman's credibility. She will be back on the stand today. She doesn't seem too pleased about that. She did admit she lied in the deposition. It was effective. They were able to show she told a couple of lies. They weren't directly related to the most important point here. But there were inconsistencies which were significant. There were outright lies, which were significant. And jurors, if they don't believe some of what a witness says, are allowed to disregard everything a witness says. So, we'll just have to see how she holds up, as this cross-examination continues. She seemed shocked -- yeah. The defense attorney really needs to discredit her testimony because it's that important. Also, jurors heard some of the nonemergency calls, some calls that zimmerman made to the police station. And how important is that? They're trying to show, this is a guy who calls the police whenever he sees a black person in his neighborhood. Now, the defense would say, that's not at all what's happening here. But that's the point. All right, dan. We'll see what happens today.

This transcript has been automatically generated and may not be 100% accurate.

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